Linear polymers of carbon monoxide and one or more .alpha.-olefins in which the monomer units from carbon monoxide and the monomer units from the olefins are present in an alternating arrangement can be prepared by contacting the monomers with a catalyst composition containing a Group VIII metal and a phosphorus bidentate ligand of the general formula ##STR1## in which R.sup.1, R.sup.2, R.sup.3, and R.sup.4 each represent identical or different monovalent aromatic or aliphatic hydrocarbon groups and R is a divalent organic bridging group containing at least two carbon atoms in the bridge connecting the two phosphorus atoms to each other. Using these catalyst compositions, linear alternating polyketone polymers are obtained with an average molecular weight, calculated as number average (M.sub.n), of more than 10,000.
The polyketone polymers, or polyketones, have repeating units of the formula ##STR2## wherein A is a unit derived from at least one olefinically unsaturated hydrocarbon. U.S. Pat. No. 4,880,903 (Van Broekhoven et al.), for example, discloses a linear alternating polyketone terpolymer of carbon monoxide, ethylene, and other olefinically unsaturated hydrocarbons, such as propylene. Processes for production of the polyketone polymers typically involve the use of a catalyst composition formed from a compound of a Group VIII metal, the anion of a strong acid, and a bidentate ligand of phosphorus, nitrogen, or sulfur. U.S. Pat. No. 4,843,144 (Van Broekhoven et al.), for example, incorporated herein by reference, discloses a process for preparing polymers of carbon monoxide and at least one ethylenically unsaturated hydrocarbon using a catalyst comprising a compound of palladium, the anion of a nonhydrohalogenic acid having a pKa of below about 6 and a bidentate ligand of phosphorus.
For some applications there is a need for polyketone polymers with a considerably lower average molecular weight. These polymers can be used as such or can serve as starting material for the preparation of other valuable polymers by chemical modification. The carbonyl groups present in the polymers as functional groups can be converted by chemical reaction at least partly into a variety of other functional groups. This chemical modification changes the properties of the polymers and they become eligible for applications for which the original polymers were unsuitable or less suitable. Chemical reactions which can be applied to the low molecular weight polymers include the conversion to polyalcohols by catalytic hydrogenation, the conversion to polypyrroles by reaction with primary amines or ammonia, the conversion to polyamines or polythiols by catalytic hydrogenation in the presence of ammonia or hydrogen sulphide respectively, the conversion to polyphenols by condensation with phenols, and the conversion to polyketals by reaction with alcohols.
The applicant has carried out an investigation into the methods for preparation of polyketone polymers with a low average molecular weight. Initially, fractionation was tried as a means to separate a low molecular weight fraction from the material prepared in the conventional manner and having a number average molecular weight of more than 10,000. Apart from the fact that this separation method was very time consuming, it produced only a very low yield of the desired low molecular weight material. Carrying out the polymerization at very high temperatures was also rejected as unattractive, since this manner of processing has a very unfavorable influence on the stability of the catalyst composition.
It was determined that the presence of hydrogen in the polymerization reactor could lead to the desired goal. The initial results from the use of catalyst compositions containing a tetraarylbisphosphine as bidentate ligand were disappointing. Although it was possible in this way to achieve some reduction in the average molecular weight, the prepared polymers still possessed a M.sub.n which was considerably higher than 10,000. It was, however, found that the presence of hydrogen has a very strong lowering effect on the average molecular weight of the prepared polymers if the polymerization is carried out using a catalyst composition containing a tetraalkylbisphosphine as bidentate ligand. In this way it was possible to prepare polymers with a M.sub.n of less than 2500 in a high yield.
Upon further investigation, it has now been surprisingly found that when using catalyst compositions containing a Group VIII metal and a tetra(sec-alkyl)bisphosphine, the desired polymers with a low molecular weight can also be prepared in the absence of hydrogen in the polymerization reactor. A tetra(sec-alkyl)bisphosphine, in which the carbon atoms forming part of the alkyl groups and linked to phosphorus are attached to two carbon atoms and only one hydrogen atom, was incorporated in the catalyst composition. By using such catalyst compositions, it was possible to prepare polymers in high yield with a M.sub.n of less than 2500, in the absence of hydrogen.